PROJECT SUMMARY/ABSTRACT The long-term objective of our research is to understand the mechanisms that cause type 2 diabetes (T2D) in relatively young Hispanic Americans in order to develop better approaches to prediction, prevention and early treatment. The specific objective of the BetaGene Study is to identify genes that predispose to T2D and understand how those genes contribute to development of diabetes. In the first five years of BetaGene, we performed oral (oGTT) and intravenous (ivGTT) glucose tolerance tests and body composition by DEXA on 1235 individuals from Mexican American families with probands who had either gestational diabetes (GDM) or normal glucose tolerance during pregnancy. In a separate cohort of Hispanic women with prior GDM, we have shown that T2D results from a progressive loss of pancreatic -cell function that occurs over the course of years on a background of chronic insulin resistance. The cross-sectional differences in -cell function that we and others have tested for association with putative T2D genes are, at best, surrogates for the more important longitudinal changes. The primary hypothesis underlying this proposal is that one or more T2D genes influence rates of change in -cell compensation for insulin resistance. We provide strong evidence for our hypothesis from preliminary studies of HNF4A. We will achieve three aims to test our hypothesis more fully. First, we will recruit and re-phenotype a random longitudinal cohort of 400 individuals from the BetaGene sample 3-5 years after their baseline exams. They will be our primary resource for association studies based on changes in -cell compensation. Second, we are already genotyping the entire BetaGene cohort for 20 genes for T2D and related quantitative traits. We will genotype the longitudinal cohort for relevant new genes underlying T2D and T2D-related phenotypes as they are discovered and for a panel of ancestrally informative markers to assess population substructure. Third, we will analyze data to test for association between variants underlying T2D and T2D-related phenotypes and rates of change in -cell compensation. We will also test for interactions between genetic effects and aspects of the -cell environment (e.g., obesity, insulin resistance, diet, physical activity) on rates of change in -cell compensation. Our results will provide unique information about genetic influences on the primary physiological abnormality that causes T2D in young Hispanic Americans. They will also provide unique information on the interplay among genetic variation and obesity, insulin resistance and -cell function. The information will help guide mechanistic studies of the genetic contribution to diabetes. It will also provide a basis for new clinical approaches to diabetes prediction, prevention and early treatment.